1. Field of the Invention
This invention relates to an apparatus for mixing thermoplastic materials, and more particularly to screws for mixing multiple polymers, which screws can be used in extruders and injection molding machines.
2. Description of the Related Art
The use of screws to extrude, compound, and injection mold polymer is well known. Turning to FIG. 1, there is shown a conventional screw 11 for use in melting and mixing polymers. The screw includes three zones: a feeding zone 13, a compression or transition zone 15 and a metering zone 17. Screw 11 is housed in a hollow barrel 19 having a constant inner cylindrical diameter and a smooth inner surface. Polymer resin, which may be in any form such as pellets, granules, flakes or powder, is fed through an opening 21 in barrel 19 into feeding zone 13 where screw 11 turns to pack and then push the polymer into compression zone 15. The polymer is melted in compression zone 15 and then conveyed to metering zone 17 where the molten material is homogenized. Afterwards, the homogenized melt is extruded, injection molded or processed further.
Screw 11 includes a screw shaft 23 having a thread spirally positioned about shaft 23 to form flights 25. Flights 25 are characterized by their depth, which is the height of flight 25 above shaft 23 and by their pitch, which is the distance P between two adjacent flights 25 plus one flight width. The outside diameter OD of screw 11 includes the depth of a flight 25 above and below shaft 23, whereas the root diameter RD of screw 11 is the diameter of shaft 23 only, without including the depth of flights 25.
U.S. Pat. No. 3,486,192 discloses a mixing element for incorporation into an extrusion screw, which mixing element is shown in FIG. 2. The mixing element 30 is an elongated cylinder with a surface that is machined with surface grooves 32 and 34 which are separated by raised lands 36. The input grooves 32 are open at the end of the mixing element facing the end of the element 30 from which polymer is fed by the screw to the mixing element and the input grooves 32 are closed at the opposite end of the mixing element. The output grooves 34 are open at the end of the mixing element facing the end of the element 30 toward which polymer is discharged and the output grooves 34 are closed at the opposite end of the mixing element. The lands 36 form a barrier between the input grooves 32 and the output grooves 34, but there is a clearance between the top of the lands and the inside surface of the barrel 19. The mixing element 30 rotates with and is driven by the turning of the screw shaft 23. The rotation of the screw forces molten polymer into the input grooves 32 of the mixing element 30, over the top of the lands 36 through the clearance between the lands 36 and the inside surface of the barrel 19 and out through the output grooves 34. The polymer melt is subjected to a high sheer stress as it is squeezed between the top of the lands 36 and the inside surface of the barrel 19. Additional screw mixing element configurations are disclosed in U.S. Pat. No. 6,136,246.
With mixing elements like that described above, it has been difficult to obtain a very high degree of polymer mixing without at the same time generating excessive heating of the polymer which frequently causes degradation of the polymer. Accordingly, there is a need for a mixing screw for use in extruders and injection molding machines that achieves a very high degree of mixing and homogenization but that does not generate undue heating or degradation of the polymer. There is also a need for a screw with a mixing element that generates a high degree of polymer mixing but does not constrain flow of the polymer through the screw so as to make reductions in screw rotation speeds and overall polymer resin throughput rates necessary. What is needed, therefore, is an apparatus that will produce a homogeneous melt from multiple polymers without causing substantial degradation of the polymer resins.
The present invention relates to an apparatus for mixing thermoplastic materials, and more particularly to screws for mixing multiple polymers which screws can be used in extruders and injection molding machines. The apparatus includes a barrel having a hollow space therein with a substantially round cross section and a longitudinal axis, with the barrel hollow space having an inside surface and a screw rotatably mounted in the barrel hollow space. The screw is mounted coaxially with the longitudinal axis of the barrel hollow space so as to transmit a polymer material through the barrel hollow space from an inlet end of the barrel hollow space to an outlet end of the barrel hollow space when the screw is rotated. The screw has a screw shaft with a thread spirally extending around the screw shaft so as to form a plurality of flights, and the screw has a zone through which melted polymer is conveyed. The screw has a flow cross-sectional area between adjacent flights of the screw that is equal to the radial height that the screw thread of the adjacent flights extends above the screw shaft multiplied by the width between adjacent flights. The screw has a portion within the zone through which melted polymer is conveyed which screw portion is without a screw thread and where the screw shaft forms a mixing element having an inlet end directed toward the inlet end of the barrel hollow space and an outlet end directed toward the outlet end of the barrel hollow space.
The mixing element of the screw comprises a drum-shaped surface extending above the screw shaft that is coaxial with the screw shaft. This drum-shaped surface has a plurality of input grooves on the drum-shaped surface extending in a generally axial direction with groove openings at the inlet end of the mixing element, wherein the input grooves terminate before reaching the outlet end of the mixing element, and a plurality of output grooves on the drum-shaped surface extending in a generally axial direction with groove openings at the outlet end of the mixing element, wherein the output grooves terminate before reaching the inlet end of the mixing element. The plurality of output grooves alternate with the plurality of input grooves such that the input and output grooves are contiguous with each other over a portion of the surface of the mixing element. Lands extending between the input grooves and output grooves separate the input grooves from the output grooves, with the length of each land being substantially equal to the length of the contiguous portion of the adjacent input and output grooves. Each of the lands has a top surface extending the length of the land that has a thickness between the input and output grooves of less than 2 mm. The top surface of the lands extends toward but does not touch the inside surface of the hollow barrel. The clearance between the top surface of said lands and the inside surface of the barrel hollow space is preferably less than 0.5 mm.
According to the invention, there is a land cross-sectional shear area corresponding to each land that is equal to length of the land multiplied by the clearance between the land and the inside surface of the barrel hollow space, and the summation of the land cross-sectional shear areas for all of the lands of the mixing element is greater than or equal to 95% of the flow cross-sectional area between the adjacent flights of the screw through which polymer passes immediately prior to entering the mixing element.